Most materials expand upon heating, with the expansion being generally unequal in each dimension. There are only a few compounds that are known to exhibit isotropic negative thermal expansion, i.e., the compounds contract equally (isotropically) in all directions upon heating. Examples of known compounds that exhibit isotropic negative thermal expansion are described in U.S. Pat. No. 5,322,559, which is incorporated herein by reference.
Zirconium tungstate (ZrW.sub.2 O.sub.8) and hafnium tungstate (HfW.sub.2 O.sub.8) are known compounds. It also is known that the expansion coefficients for zirconium tungstate are negative. Martinek, et al.'s Linear Thermal Expansion of Three Tungstates, J. Am. Ceram. Soc. Discussions and Notes, 51:227-228 (1968). The benefits deriving from the thermal expansion characteristics of these tungstate compounds have gone unrecognized. Moreover, as discussed in more detail below, known procedures for making zirconium tungstate are less than optimal.
There are five known literature reports for the synthesis of zirconium tungstate. Each of these procedures used zirconium(IV)oxide (ZrO.sub.2) and tungsten(VI)oxide (WO.sub.3) as the reactants. The synthetic procedures involve first comminuting the reactants (reducing the reactants to small particles) and then heating them together to a temperature of about 1200.degree. C. The step of heating is then followed by rapid cooling to prevent zirconium tungstate from decomposing into zirconium oxide and tungsten oxide.
Graham originally stated that the reaction of zirconium oxide with tungsten oxide to form zirconium tungstate was complete within about 15 minutes at 1200.degree. C. The authors later recanted the success of the synthetic procedure, and stated that they were never able to prepare zirconium tungstate that was free of zirconium oxide and tungsten oxide. Graham et al.'s A New Ternary Oxide, ZrW.sub.2 O.sub.8, J. Am. Ceram. Soc., Discussions and Notes, 42:570-571 (1959). This difficulty is apparently related to (1) the volatility of tungsten oxide at 1200.degree. C., and (2) the reactivity, or lack thereof, of zirconium oxide under the synthetic conditions used by Graham.
Martinek and Hummel also described the synthesis of certain tungstate compounds, including zirconium tungstate. Martinek et al.'s, Linear Thermal Expansion of Three Tungstates, supra. The synthesis involved reacting zirconium oxide with "tungstic acid," which is either H.sub.2 WO.sub.4 or WO.sub.3. It is unclear from Martinek's disclosure which of these compounds was used. The reactants were combined and then heated to a temperature of about 1150.degree. C., which was maintained for four hours, followed by rapid cooling. There is no indication in this paper concerning the purity of the compound that was obtained by the synthetic procedure described.
Chang also described a method for preparing zirconium and hafnium tungstate. Chang et al.'s Condensed Phase Relations in the Systems ZrO.sub.2 --WO.sub.2 --WO.sub.3 and HfO.sub.2 --WO.sub.2 --WO.sub.3, J. Am. Ceram. Soc., 50:211-215 (1967). Chang placed the respective reactants in sealed platinum tubes, primarily because of the volatility of WO.sub.3 at elevated temperatures. Chang specifically states that "equilibrium experiments indicate that this compound zirconium tungstate! can only be obtained as a single phase from ZrO.sub.2 and WO.sub.3 in the proper stoichiometric ratio through prolonged heating, i.e., at least 24 hr. at 1200.degree. C." Chang et al., supra, at page 212. Martinek later refuted this statement. Martinek specifically stated that the "present work only confirmed the limited stability of ZrW.sub.2 O.sub.8. The compound cannot be formed below 1105.degree. C., and even at 1200.degree. C. 48 h were required to produce ZrW.sub.2 O.sub.8 free of ZrO.sub.2 and WO.sub.3." Martinek et al.'s Subsolidus Equilibria in the System ZrO.sub.2 --WO.sub.3 --P.sub.2 O.sub.5, J. Am. Ceram. Soc., 53:159-161 (1970) .
In summary, there are significant drawbacks associated with known synthetic methods for making zirconium and hafnium tungstates. For instance, the only reactants used in procedures to synthesize zirconium or hafnium tungstate are zirconium(IV)oxide, hafnium(IV)oxide and tungsten(VI)oxide. This substantially limits the synthetic approaches to forming tungstates. Furthermore, it appears that the known methods produce compounds that are contaminated with other materials, even after prolonged, and therefore commercially expensive, heating periods.